Methods have been developed for transferring functional genes into mammalian cells. A primary technique is physical microinjection of specific cloned genes into the nucleus of individual tissue culture cells and into mouse fertilized eggs. Mice containing microinjected clones genes in their genomic DNA have been obtained and have transferred the DNA through sevaral generations. Treatment of microinjected mice with 5-azacytidine results in partial activation of the exogenous gene. A mouse model for Beta-thalassemia has been characterized.